This invention relates generally to the manufacture of lead pencils, and more particularly to a technique for fabricating pencils having a sheath formed of foam plastic material.
The conventional pencil consists of a core or lead locked within a sheath or casing made of wood. The lead is a mixture of clay and graphite, the clay serving as a binder. The hardness of the lead is determined by the ratio of clay to graphite in the mixture. The wood must be of a special quality in that in the finished pencil, the sheath not only acts as the support for the relatively fragile lead, but it must also lend itself readily to sharpening.
The technique for manufacturing pencils with wooden sheaths and lead cores is costly and time-consuming, for not only are at least 12 steps involved in the manufacturing procedure, but a large quantity of wood is wasted. With the increasing shortage of wood acceptable for pencils, the waste factor becomes less tolerable.
In the standard method of making lead pencils, a wood block, usually of cedar, is first cut into small slabs which are then provided with an array of parallel grooves to accommodate the leads. A second grooved slab is placed over the first slab to enclose the leads, the two slabs being then glued together. Thereafter the joined slab assembly is fed through a cutting machine to produce individual pieces which are thereafter shaped into the familiar hexagonal or round forms. Finally the pencils are painted and erasers are installed at the ends.
Among the problems encountered with conventional pencils having wood sheaths are the following:
A. Pencils are exposed to varying degrees of temperature and humidity, causing the wood to expand or dry out as a result of which the lead loosens in the sheath and slips out and breaks. This is especially characteristic of pencils formed of two wood slabs having different grains.
B. When the wood sheath is manually sharpened, the grain of the wood tends to guide the knife, and in some cases the persons sharpening the pencil cannot control the direction of cutting in the desired manner. And with machine-sharpened wood pencils, the sharpened point is left rough because the cutting blades operate across the grain.
C. Cedar for wood pencils is now difficult to obtain and what cedar is available is quite expensive.
In order to reduce the cost of manufacturing lead pencils, attempts have been made to substitute a moldable material for the wooden sheaths. Thus in U.S. Pat. No. 1,649,892, a pulp mass is extruded simultaneously with the advance of a central lead to form a pencil. This process requires subsequent evaporation of the moisture content of the extruded pulp to render the body rigid and inflexible. In a similar fashion, U.S. Pat. No. 1,937,104 forms a sheath of wood flour and a water-soluble binder.
It is also known, as evidenced by the U.S. Pat. Nos. 2,960,967 and 3,551,064, to form the sheath of extruded plastic material. And while the substitution of synthetic plastic for wood overcomes the problem of wood shortages, pencils of the type theretofore known have a serious disadvantage for there is not an adequate bond between the lead and the plastic sheath; hence the lead has a tendency to slip out and break.